This application is a U.S. national phase of International Application PCT/F199/00043 filed Jan. 22, 1999.
The present invention relates to a method of fabricating paperboard cores for the paper industry, said paperboard cores having an improved chuck strength and thick walls, the wall thickness H being 10 mm or more and the inside diameter over 70 mm. Such cores are used at winding/unwinding speeds of at least about 200 m/min (3.3 m/s). The invention also relates to a method of fabricating other paperboard cores of similar dimensions, which call for high chuck strength. The invention further relates to a spirally wound, thick-walled core constructed by this method.
Cores used by the printing and paper converting industries are herein referred to as paper industry cores. Such cores are thick-walled, having a wall thickness H which is at least 10 mm and an inside diameter which is over 70 mm.
A spiral paperboard core is made up of a plurality of superimposed plies of paperboard by winding, glueing, and drying such.
Webs produced in the paper, film, and textile industries are usually reeled on cores for rolls. Cores made from paperboard, especially spiral cores are fabricated by glueing plies of paperboard one on top of the other and by winding them spirally in a special spiral machine. The width, thickness, and number of paperboard plies needed to form a core vary depending on the dimensions and strength requirements of the core to be manufactured. Typically, the ply width is 50 to 250 mm (in special cases about 500 mm), ply thickness about 0.2 to 1.2 mm, and the number of plies about 3 to 30 (in special cases about 50). The strength of a paperboard ply varies to comply with the strength requirement of the core. As a general rule, increasing the strength of a paperboard ply also increases its price. Generally speaking, it is therefore true to say that the stronger the core, the more expensive it is.
In the paper converting industry, weights of paper rolls used, e.g., in printing presses have been on a continuous increase, which calls for a higher and higher strength and a higher and higher capacity of spiral cores. The weights of paper rolls vary considerably, from newspaper and fine paper rolls of 600-1800 kg to rotogravure rolls of about 2400-5500 kg. The biggest rolls that have been made, for testing purposes, have weighed about 6500 kg. The diameters of big paper rolls are then typically 1.24 to 1.26 m at most.
Printing presses typically use cores of two sizes. The most usual core size has the inside diameter of 76 mm (3xe2x80x3) and the wall thickness of 13 or 15 mm. Today, the widest and fastest printing presses, i.e., those with the heaviest rolls, use cores with the inside diameter of 150 mm (6xe2x80x3) and normally the wall thickness of 13 mm.
Printing presses are being designed which should handle paper rolls having a diameter of 1.35 m; estimates have been presented of even 1.5 m rolls. As the roll width increases to 3.6 m, the weight of a paper roll will increase considerably, to more than 6.5 tons, even to 8.5 tons.
Typical ply widths of paperboard cores used in the printing and paper converting industries, as discussed above, are about 120 to 150 mm with cores having the inside diameter of 76 mm (3xe2x80x3), which is the most commonly used inside diameter, and up to 190 mm with cores having the inside diameter of 150 mm (6xe2x80x3). Due to core geometry, average winding angles a then range from about 15 to about 35xc2x0, depending on the core diameter. The wall thicknesses of paperboard cores are typically about 10 to 20 mm. The definition of the average winding angle xcex1 is presented in FIG. 3 below.
Paper reels are formed on a winding core. Almost always this winding core is a spirally wound paperboard core.
The requirement of a good chuck strength is emphasized especially in, e.g., shaftless winding/unwinding of a paper web, where the core, serving as the only shaft, bears the weight of the paper roll either partly or completely through short chucks of about 50 to 250 mm in length. Furthermore, the chuck may be subject to a pressure of accelerating belts needed for an automatic reel change in the printing press. These accelerating belts may cause an extra strain of even 1 to 2 tons on the core.
The chuck strength is an essential requirement at the paper mill in making the roll, when slitter winders of the so-called centre winder type are used.
In shaftless winding and unwinding, the weight of a paper roll creates stresses in the core, at chucks. The most dangerous of them are shear stresses and radial stresses.
When paper rolls equal in weight are supported, these stresses become different as to their form and extent, depending on the wall strength and inside diameter of the core. The form of stresses at different points inside the core wall as well as the point where the maximum stresses occur, may be calculated, and it may also be found experimentally, e.g., by using a method and apparatus in accordance with European patent 309 123.
As discussed above, cores are subject to different stresses when they are used, e.g., in a paper roll. In shaftless winding/unwinding, the core serves as the only shaft, supporting the weight of the paper roll either entirely or partly, through a short chuck. The pressure caused by accelerating belts, needed for automatic reel change at printing presses, possibly adds to the weight.
In this kind of a situation, the core becomes subject to several stresses, which strain the core and may cause its breakage. As a paperboard core is an orthotropic material, knowing these stresses is a highly exacting task.
By using advanced modelling methods known to a person skilled in the art, shear, compressive or flat crush, and tensile stresses may be analysed so as to find out where different stresses appear, and also, at which depth in the core wall there are stresses in actual use and how heavy they are. The results of the analysis may be confirmed experimentally, e.g., by using a method and apparatus in accordance with EP patent 309 123. By using the test method in accordance with EP 309 123, it is possible to simulate stresses of a core in use conditions. These stresses, appearing in use conditions, may also be modelled by means of computationally demanding finite-element methods. We have made stress analyses of chuck loading, which have indicated and experimental testing (by using an apparatus according to EP 309 123) confirmed that the heaviest z-direction stresses appear almost in the middle of core wall, slightly towards the inner surface of the core. The z-direction here means a direction perpendicular to the surface level of a paperboard ply, i.e., in the cross section of a finished core, it is the direction of the core radius.
The z-direction maximum tensile and shear stresses directed to the plies are radial, occurring near the middle of the core wall, slightly inwardly therefrom.
We have described the problematic area which our invention originates from. A review of prior art revealed U.S. Pat. No. 3,194,275. The problems treated there are, however, totally different and the solution provided is completely different from ours. U.S. Pat. No. 3,194,275 will be discussed further below, in connection with the more detailed description of the present invention. The comparison between the present invention and the arrangement disclosed in U.S. Pat. No. 3,194,275 indicates that the problems and, consequently, their solutions are different from each other.
An object of the present invention is to provide an improved and more efficient method of fabricating thick-walled paperboard cores for the paper industry, the wall thickness being over 10 mm and the inside diameter over 70 mm.
Another object of the present invention is to provide an improved method of increasing the chuck strength of both thick-walled paperboard cores for the paper industry, which have the wall thickness of over 10 mm and inside diameter of over 70 mm, and other paperboard cores which require high chuck strength, and at the same time to provide a novel type of thick-walled spiral paperboard core which has better properties in use.
A further object of the present invention is to solve problems related to the above discussed thick-walled spiral cores presently in use and to offer a solution for meeting the requirements set by ever increasing roll weights, especially on the chuck strength of cores.
These objects are achievable by the arrangement in accordance with the accompanying claims.
As discussed above, typical wall thicknessxe2x80x94inside diameter figures are, e.g., 15 mmxc3x9776 mm and 13 mmxc3x97150 mm. Stresses caused by chuck loading on the biggest cores, such as, e.g., 13 mmxc3x97300 mm (10 mmxc3x97300 mm) are naturally lower than on paper industry cores having a smaller diameter, due to the core geometry. Thus, the chuck strength of, for example, 13xc3x97300 mm core is in itself higher than the chuck strength of cores having a small diameter. This is because, due to a big inside diameter, the bearing area of the core with respect to the shaft is large. The present invention does not relate to paperboard cores which have a wall thickness less than 10 mm. Paper industry cores must have a thick wall, i.e., more than 10 mm in order to enable them to be clamped by chucks (chuck expansion) and in order to enable formation of a nip between the core surface and a backing roll. Especially, the geometry of winders and slitter-winders calls for a sufficient wall thickness of cores, 10 mm or more, in practice. The arrangement of the present invention increases the production rate of all paper industry cores with different diameters, but its advantages as to the increase of chuck strength is pronounced with paper industry cores of small diameters. The greatest significance of an improved chuck strength is established in connection with most commonly used cores which have the inside diameter of 3xe2x80x3 (about 76 mm). A significant improvement of the chuck strength is achieved also with cores having the inside diameter of 6xe2x80x3 (about 150 mm).
The arrangement according to the present invention is also applicable to the fabrication of other paperboard cores, which require high chuck strength and which have similar dimensions as the cores according to the present invention, used in the printing and paper converting industries.
The present invention deals with core breaking, caused by a crack breaking mechanism. When breakages in cores occur in the paper industry, this is the most frequent mechanism, in practice. Here, the break of a core occurs in the cylindrical surface within the core wall and/or in the vicinity thereof, in which cylindrical surface the maximum stresses are to be found. Therefore, we have presented the widths and web edge lengths of the core ply on the level of the cylindrical surface and in the vicinity thereof, as attributes describing our invention. In principle, corresponding definitions could be made with respect to the interior or exterior plies, the dimensions of which are determined by selecting the structural dimensions of the core and by fixing, on the maximum stress surface, the ply length per linear meter of core or the ply width.
It is therefore an essential object, according to the present invention, that especially on the cylindrical surface representing the maximum stress in the wall direction of the cross section, i.e., z-direction of the core, but also elsewhere in the core wall, there are as few potential points for initial cracks as possible, which would lead to a breakage. By influencing potential points of initial cracks, i.e., by reducing their number, it is possible to influence particularly the chuck strength (delamination strength) of the core, i.e., to increase it.
The arrangement according to the invention, for improving the chuck strength of thick-walled paperboard cores for the paper industry, makes use of, e.g., the following discoveries.
With narrow plies, only a small pitch is formed per linear meter of the core, whereby there are several gaps between the plies per length unit of the core. Widening of the paperboard ply reduces the length of gaps per linear meter of the core.
The basic idea of our invention is to reduce the length of the gaps per linear meter of the core, thereby providing a paper industry core, which has less than before of web edge line of ply per linear meter, i.e., fewer potential points of initial cracks per linear meter of the core than before.